We present the X-ray properties of 108 Dust-Obscured Galaxies (DOGs; F24 m/FR > 1000) in the COSMOS field, all of which are detected in at least three far-infrared bands with the Herschel ...Observatory. Out of the entire sample, 22 are individually detected in the hard 2-8 keV X-ray band by the Chandra COSMOS Legacy survey, allowing us to classify them as AGN. Six (27%) of them are Compton-thick AGN candidates with column densities NH > 1024 cm−2, while 15 are moderately obscured AGNs with 1022 < NH < 1024 cm−2. Additionally, we estimate AGN contributions to the IR luminosity (8-1000 m rest-frame) greater than 20% for 19 DOGs based on SED decomposition using Spitzer/MIPS 24 m and the five Herschel bands (100-500 m). Only 7 of these are detected in X-rays individually. We performed an X-ray stacking analysis for the 86 undetected DOGs. We find that the AGN fraction in DOGs increases with 24 m flux and that it is higher than that of the general 24 m population. However, no significant difference is found when considering only X-ray detections. This strongly motivates the combined use of X-ray and far-IR surveys to successfully probe a wider population of AGNs, particularly for the most obscured ones.
We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation ( Delta *SSFR and SFRIR/SFRUV) on the position of galaxies in the star formation rate (SFR) versus ...mass diagram. Our sample comprises roughly 640,000 galaxies at z ~ 0.1, 130,000 galaxies at z ~ 1, and 36,000 galaxies at z ~ 2. Structural measurements for all but the z ~ 0.1 galaxies are based on Hubble Space Telescope imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a 'Hubble sequence') is already in place since at least z ~ 2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid buildup of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Delta *SSFR at each redshift. A simple model using an empirically determined star formation law and metallicity scaling, in combination with an assumed geometry for dust and stars, is able to relate the observed Delta *SSFR and SFRIR/SFRUV, provided a more patchy dust geometry is assumed for high-redshift galaxies.
We investigate the star formation properties of a large sample of ∼2300 X-ray-selected Type 2 Active Galactic Nuclei (AGNs) host galaxies out to in the Chandra COSMOS Legacy Survey in order to ...understand the connection between the star formation and nuclear activity. Making use of the existing multi-wavelength photometric data available in the COSMOS field, we perform a multi-component modeling from far-infrared to near-ultraviolet using a nuclear dust torus model, a stellar population model and a starburst model of the spectral energy distributions (SEDs). Through detailed analyses of SEDs, we derive the stellar masses and the star formation rates (SFRs) of Type 2 AGN host galaxies. The stellar mass of our sample is in the range of with uncertainties of ∼0.19 dex. We find that Type 2 AGN host galaxies have, on average, similar SFRs compared to the normal star-forming galaxies with similar Mstellar and redshift ranges, suggesting no significant evidence for enhancement or quenching of star formation. This could be interpreted in a scenario, where the relative massive galaxies have already experienced substantial growth at higher redshift ( ), and grow slowly through secular fueling processes hosting moderate-luminosity AGNs.
We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24 μm observations of the COSMOS field at S 24 μm gsim 80 μJy. Using accurate photometric redshifts (σ ...z ~ 0.12 at z ~ 2 for 24 μm sources with i + lsim 25 mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24 μm background intensity originate from galaxies at z lsim 1 and z lsim 2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z gsim 2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i +-band sources with 24 μm detection strongly increases up to z ~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z ~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24 μm. Furthermore, the simulated galaxies at S 24 μm > 80 μJy exhibit R-K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z gsim 0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24 μm sources up to z ~ 3, although none of them is able to exactly match our results at all redshifts.
Abstract
We set out to quantify the number density of quiescent massive compact galaxies at intermediate redshifts. We determine structural parameters based on i-band imaging using the ...Canada–France–Hawaii Telescope (CFHT) equatorial Sloan Digital Sky Survey (SDSS) Stripe 82 (CS82) survey (∼170 deg2) taking advantage of an exquisite median seeing of ∼0.6 arcsec. We select compact massive (M
⋆ > 5 × 1010 M⊙) galaxies within the redshift range of 0.2 < z < 0.6. The large volume sampled allows to decrease the effect of cosmic variance that has hampered the calculation of the number density for this enigmatic population in many previous studies. We undertake an exhaustive analysis in an effort to untangle the various findings inherent to the diverse definition of compactness present in the literature. We find that the absolute number of compact galaxies is very dependent on the adopted definition and can change up to a factor of >10. We systematically measure a factor of ∼5 more compacts at the same redshift than what was previously reported on smaller fields with Hubble Space Telescope (HST) imaging, which are more affected by cosmic variance. This means that the decrease in number density from z ∼ 1.5 to z ∼ 0.2 might be only of a factor of ∼2–5, significantly smaller than what was previously reported. This supports progenitor bias as the main contributor to the size evolution. This milder decrease is roughly compatible with the predictions from recent numerical simulations. Only the most extreme compact galaxies, with R
eff < 1.5 × (M
⋆/1011 M⊙)0.75 and M
⋆ > 1010.7 M⊙, appear to drop in number by a factor of ∼20 and hence likely experience a noticeable size evolution.
We set out to quantify the number density of quiescent massive compact galaxies at intermediate redshifts. We determine structural parameters based on i-band imaging using the Canada–France–Hawaii ...Telescope (CFHT) equatorial Sloan Digital Sky Survey (SDSS) Stripe 82 (CS82) survey (∼170 deg^2) taking advantage of an exquisite median seeing of ∼0.6 arcsec. We select compact massive (M_⋆ > 5 × 10^10 M_⊙) galaxies within the redshift range of 0.2 < z < 0.6. The large volume sampled allows to decrease the effect of cosmic variance that has hampered the calculation of the number density for this enigmatic population in many previous studies. We undertake an exhaustive analysis in an effort to untangle the various findings inherent to the diverse definition of compactness present in the literature. We find that the absolute number of compact galaxies is very dependent on the adopted definition and can change up to a factor of >10. We systematically measure a factor of ∼5 more compacts at the same redshift than what was previously reported on smaller fields with Hubble Space Telescope (HST) imaging, which are more affected by cosmic variance. This means that the decrease in number density from z ∼ 1.5 to z ∼ 0.2 might be only of a factor of ∼2–5, significantly smaller than what was previously reported. This supports progenitor bias as the main contributor to the size evolution. This milder decrease is roughly compatible with the predictions from recent numerical simulations. Only the most extreme compact galaxies, with R_eff < 1.5 × (M_⋆/10^11 M_⊙)^0.75 and M_⋆ > 10^10.7 M_⊙, appear to drop in number by a factor of ∼20 and hence likely experience a noticeable size evolution.
We present the results of a Herschel-PACS study of a sample of 97 low-ionization nuclear emission-line regions (LINERs) at redshift z ~ 0.3 selected from the zCOSMOS survey. Of these sources, 34 are ...detected in at least one PACS band, enabling reliable estimates of the far-infrared L sub(FIR) luminosities, and a comparison to the FIR luminosities of local LINERs. Many of our PACS-detected LINERs are also UV sources detected by GALEX. Assuming that the FIR is produced in young dusty star-forming regions, the typical star formation rates (SFRs) for the host galaxies in our sample are ~10 M sub(middot in circle) yr super(-1), 1-2 orders of magnitude larger than in many local LINERs. Given stellar masses inferred from optical/NIR photometry of the (unobscured) evolved stellar populations, we find that the entire sample lies close to the star-forming "main sequence" for galaxies at redshift 0.3. For young star-forming regions, the H alpha - and UV-based estimates of the SFRs are much smaller than the FIR-based estimates, by factors ~30, even assuming that all of the Ha emission is produced by O-star ionization rather than by the active galactic nuclei (AGNs). These discrepancies may be due to large (and uncertain) extinctions toward the young stellar systems. Alternatively, the H alpha and UV emissions could be tracing residual star formation in an older, less obscured population with decaying star formation. We also compare L sub(SF) and L(AGN) in local LINERs and in our sample. Finally, we comment on the problematic use of several line diagnostic diagrams in cases with an estimated obscuration similar to that in the sample under study.
We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24 mum observations of the COSMOS field at S 24 mum 80 muJy. Using accurate photometric redshifts ...(sigma z ~ 0.12 at z ~ 2 for 24 mum sources with i + 25 mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24 mum background intensity originate from galaxies at z 1 and z 2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z 2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i +-band sources with 24 mum detection strongly increases up to z ~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z ~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24 mum. Furthermore, the simulated galaxies at S 24 mum > 80 muJy exhibit R-K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z 0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24 mum sources up to z ~ 3, although none of them is able to exactly match our results at all redshifts.
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